Renewable energy from biomass (bioenergy) can mitigate anthropogenic CO2 emissions due to reduced use of fossil energy. Cultivation of microalgae for bioenergy could be a superior and sustainable alternative to terrestrial energy crops, due to the fast growth rates of microalgae as well as their ability to grow on waste waters and marginal lands. While the potential of microalgae has been well-appreciated, present methods of cultivation pose significant hurdles in the way of economical production. Two methods of cultivation are closed photo-bio reactors and open-pond systems. Of these, open-pond systems are robust for large-scale algal cultivation.
Microalgae cultivation in open ponds is usually attempted in an autotrophic mode (i.e., photosynthetic carbon fixation) using mesophiles (viz., algae that grow in a near neutral pH environment). To achieve high photosynthesis rates, availability of dissolved inorganic carbon (DIC) (i.e., dissolved CO2 and HCO3−) is generally crucial apart from light. Unfortunately, under mesophilic conditions, slow kinetics of atmospheric CO2 absorption lead to limited DIC availability for biomass growth. Consequently, to increase the DIC, different approaches have been attempted. One of these approaches involves sparging raw flue gas or more concentrated CO2 into the ponds. Providing concentrated CO2 (either as flue gas or more concentrated CO2) further for algae culture proves to be expensive, due to the high costs of CO2 capture at the emission source using absorbents, regeneration of the absorbents, CO2 transportation to algal ponds, the costs associated with its temporary storage, and incomplete uptake by the open pond culture medium.
Some alternatives to this approach involve contacting the sorbent solution containing the absorbed CO2 with the open pond culture medium directly to strip the DIC into the culture, thus achieving cost reductions through elimination of sorbent regeneration and CO2 storage steps. However, a drawback to these approaches is that they are constrained by (i) proximate availability of flue gas or other high concentration CO2 sources, and (ii) the energy and infrastructure burden to deliver CO2 over long distances, as well as its distribution into the pond-medium. It has been estimated that microalgae cultivation systems that are constrained by the availability of flue gases (in addition to low-slope barren lands and favorable climates) could achieve less than 10% of the Department of Energy's 2030 advanced fuel targets. In addition, it is believed that nearly 65% of cultivation-related variable operating costs are associated with recovery of CO2 from flue gas and delivery to ponds (of a total operating cost of $144 per ton of dry algae, approximately $91 are attributable to CO2 delivery to ponds). In terms of overall costs of cultivation (excluding harvesting costs, but including costs to service capital for pond construction), CO2 supply contributes nearly $100 to the minimum biomass selling price (MBSP) of $400/ton of dry algae.
When “high-value” algae-based end-products are targeted (instead of fuel), an alternate strategy that could be justified is mixotrophic cultivation (i.e., supplementing CO2-derived inorganic carbon with organic carbon such as glucose) to improve the biomass yield. However, in open pond cultivation systems, mixotrophic mode cultivation raises additional issues. For example, at the pH conditions conducive for mesophilic algal growth, simultaneous growth of predatory micro-organisms is also supported by the organic carbon source, leading to algae “culture-crash”. Thus, there is a need for new and improved methods and systems for the culturing of algae.